bopper:
if you can, just call the guest and say "I just heard that you got engaged! I wanted to let you know that Darling is invited too. I can't wait to hear about the proposal!"

LadyR:
When i got married, we decided no "and guest" but friend in established relationships could bring their partner, regardless of living situation. My friend Jake was single when we made up our guest list, but at some point between then and him receivng the invation, he had begun a relationship. He asked if he could bring her, i said no, as our numbers ere really tight and it was a new relationship and i hadn't met her. He was ok with it. Flash forward three years later and they're engaged and we think she's wonderful. We really regret not extending the invite to her. In this case, the couple is already established and engaged, i'd definitely extend the invite.

Winterlight:
I think I'd do my best to invite the partner unless there was just no way (a week before my wedding when there is no room left.)

bah12:
This is such a hard question to answer because it is so situational. First, I can't imagine that I would distinguish between serious relationships based on whether or not a couple was living together. So, I'd either invite married/engaged couples only or couples in serious enough relationships that they would be a considered a social unit regardless of their living situation. And more likely the latter would be my default.

As for what to do when a couple does become engaged after invites went out, it would also depend on how close the event is and whether or not I could reasonably accommodate another guest. I don't think the snubbed/not snubbed thing is even an issue as I would imagine if one half of a serious couple was invited to an event, that the partner would realize that they weren't invited. If the rules were acceptable to them before they got engaged (and they are acceptable rules), then surely a change of status and a subsequent invite wouldn't suddenly offend them.

So, assuming that I could reasonably accommodate another person, I would call them up and congratulate them on their engagement and ask if they'd like to come to my event.

kareng57:
I can kind of see it happening, if the guest is someone that the HC doesn't have frequent contact with - an out-of-town cousin, perhaps. Some people are fairly quiet about relationships, even when they've become quite serious.

I think this is one of the pitfalls of sending invitations out too early - peoples' life circumstances do change.